1. Field of the Invention
This invention relates generally to drilling of lateral wells into an oil rim accumulation or reservoir, and more particularly to the identification of the optimum vertical position for drilling such wells.
2. Description of the Related Art
Thin oil rim accumulations positioned between gas above and water below are difficult reservoirs to produce due to the tendency of water and gas to break through. Production of such accumulations from horizontal wells improves the ultimate recovery because the resulting increase in well productivity reduces the drawdown, and thereby reduces the coning of unwanted gas and water.
Known reservoir simulations can be used to estimate the optimum vertical position of a horizontal drainhole above the water contact and below the gas contact. Drilling a lateral well at this optimum drainhole position is difficult because geometric positioning during directional drilling is achieved with imperfect surveying instruments.
SPE Paper No. 50072 entitled xe2x80x9cGeosteering Horizontal Wells in a Thin Oil Column,xe2x80x9d describes a method of horizontal drainhole positioning above the oil-water contact layer using resistivity determination. The resistivity directly above the oil-water contact zone will increase as the water saturation decreases to the irreducible value. This will occur over a transition zone.
The shape and height of this transition zone is characterized by a capillary pressure curve, which is a function of porosity and lithology. An empirical algorithm may be developed from offset-near-vertical well logs that relates the resistivity response to height above the oil-water contact for a range of porosities and clay contents.
There are several problems with this approach. For example, the resistivity value at a fixed distance above the oil-water contact is not unique. A range of such resistivity values exists depending on the formation porosity and lithology. Thus, in order to apply this technique, multiple formation measurements are required.
Another problem results from the fact that resistivity measurements are typically focussed perpendicular to the tool axis. Focussed resistivity measurements recorded in a near-vertical well will be dominated by the bed parallel resistivity, while focussed resistivity measurements taken in a near-horizontal well will be a combination of bed parallel and bed perpendicular resistivity. Thus, if resistivity anisotropy is present, it must be accounted for to apply an algorithm derived from vertical wells.
To address these shortcomings, it is a principal object of the present invention to provide formation pressure-versus-depth data for a subsurface formation that is useful for predicting the presence and depth of an oil reservoir. The formation pressure and gradient is established from offset near vertical wells, and used to relate formation pressure to absolute depth. This pressure gradient has been used to determine the vertical position of a completed well whose wellbore pressure is at equilibrium with the formation pressure by relating the wellbore pressure measured with a wireline production logging tool to the vertical height.
The object described above, as well as various objects and advantages, are achieved by a method that includes the steps of deploying a plurality of data sensors at discrete depths in a subsurface formation penetrated by a wellbore, gathering formation pressure data for the discrete depths using the data sensors, and determining a desirable depth for drilling a horizontal well within a reservoir using the gathered formation pressure data.
The formation pressure is gathered using receivers for receiving the formation pressure data transmitted by the data sensors. The receivers may be disposed within a downhole tool, and may be part of a drill string or part of a wireline sonde.
The depth within the reservoir may be determined by identifying from the gathered formation pressure data at least one depth whose corresponding formation pressure is suggestive of a reservoir. Once such a depth is identified, the wellbore itself or a lateral drainhole depending from the wellbore may be steered into the reservoir by maintaining the trajectory of the wellbore or drainhole at a substantially constant distance from a fluid contact within the reservoir.
In a preferred embodiment, the vertical depth within the reservoir is determined by comparing the gathered formation pressure data with a pre-determined formation pressure gradient. The pre-determined formation pressure gradient is established from vertical or near vertical offset wells using wireline formation pressure measurements, or from a near vertical section of the wellbore using Logging-While-Drilling (xe2x80x9cLWDxe2x80x9d) formation pressure measurements.